Water Storage Change Of Lake Victoria Basin From GRACE Gravity Measurements

Terrestrial water storage(TWS)is an important component of the global water cycle and an important resource for human survival and development.With the water scarcity becomes more severely,researches on the balance of the water cycle system are becoming more and more important.Traditional terrestrial water monitoring methods have limited spatial distribution and uneven data acquisition.GRACE(Gravity Recovery and Climate Experiment)Satellites provide a new method for monitoring global water resources changes,which can continuously monitor global gravity changes on a larger scale,and then global or regional terrestrial water change signals can be derived.Lake Victoria,the largest freshwater body in Africa,it provides the living water resources for the surrounding residents,so it's significant to monitor the terrestrial water storage change in the basin.Its drainage area is approximately261800km~2,the catchment area is enough so the gravity signal changes can be detected by GRACE,and the basin experienced significant periods of drought and wetting during the GRACE operation,so using GRACE time-varying gravity field to detect changes in its water reserves has practical significance and feasibility.In this paper the time series variation of the water storage in Lake Victoria Basin has been calculated using the time-variable gravity data from GRACE between January 2003 and December 2016.Due to the poor performance of the GLDAS(Global Land Data Assimilation System)hydrological model in the area,the effect of the scaling factor method which is commonly used to correct spatial leakage errors is not satisfactory,therefore a fully unconstrained global forward modeling method is adopted for correcting spatial leakage errors.Simulation result of lake surface height data obtained by satellite altimetry indicate that the forward modeling method is better than the scaling factor in correcting the leakage error in the study area.The new assessments based on the terrestrial water storage that is restored using the forward modeling method indicates that during the 14 year period from 2003 to 2016 the Lake Victoria Basin terrestrial water storage shows an overall upward trend,the newly estimated increased rates are 12mm/yr,and detected the trend of increasing terrestrial water storage in the basin from 2013 to 2016 at a rate of 24mm/yr.The change from 2013 to 2016 was mainly caused by the increase in rainfall.The Lake Victoria Basin terrestrial water storage is currently in surplus.There is a good consistency between GRACE terrestrial water storage and satellite altimeter measurement and GLDAS result.The groundwater changes in the basin are estimated,the result indicates that there is no obvious change trend of groundwater from 2003 to 2016,and the terrestrial water storage change signal in this area is mainly caused by surface water and soil water.The amount of evaporation in the basin is estimated,the monthly evaporation calculated using the GRACE gravity field model is approximately between 70mm and 120mm,the valuation is in good agreement with LandFlux-EVAL model results.During the period from2014 to 2016,the monthly evaporation capacity increased at a rate of 7.5mm/yr,during this period,the increase in temperature is the reason for this change.Combined with the comparison of TRMM rainfall and GRACE terrestrial water storage,the results show that the change of rainfall is consistent with the change of terrestrial water storage in Lake Victoria Basin.The rainfall from 2013 to 2016 shows an increasing trend at the rate of 6mm/yr.The ENSO(El Ni?o-Southern Oscillation)and IOD(Indian Ocean Dipole)phenomenon will have some impacts on the terrestrial water storage in the Lake Victoria Basin.The El Ni?o phenomenon from 2014 to 2016 and the positive IOD phenomenon around 2016are associated with the increased rainfall,which in turn causes changes in terrestrial water storage.